US3328211A - Method of manufacturing weldable, tough and high strength steel for structure members usable in the ashot-state and steel so made - Google Patents

Description

3,328,211 Patented June 27, 1967 ice 3,328,211 METHGD F MANUFACTURENG WELDAEBLE, TUUGH AND HlGH STRENGTH STEEL FGR STRUCTURE MEMBERS USABLE IN THE AS- HQT-STATE AND d'iEEL $0 MADE Haiime Nahamura, Megurou, Tokyo-to, Japan, assignor to lshikawajimwfi'arima .bulrogyo Kahushild Keisha, Tokyo-to, Japan, a company ct lapan No Drawing. Filed Nov. 18, 1904, Ser. No. 412,231 Claims riority, application .lapan, Dec. 5, 1053, 38/6 ,401 Claims. (Cl. 140-32) This invention relates to a weldable, tough and high strength steel for structure members usable in the as-hotworked state and method of manufacture therefor.

There are a number of steels available today on commercial market that purport to have a tensile strength of 60 kg./mm. class together with ductility, toughness and weldability that are adequate for fabrication of large steel structures such as ships, buildings, pressure vessels or bridges. Here, a term 60 kg./mrn. class is used to designate a range for tensile strength which is defined as between 58 kg/mm. and 70 kg./mm. both limits inclusive, in accordance with Japanese standards.

However, I am, as yet, unaware of one that is capable of developing the desired set of mechanical properties, namely high strengths, both yield and tensile, coupled with good ductility and/or toughness without sacrificing one in favor of the other, if left in the as-worked, for example, as-rolled state. Therefore, heat treatment of one variety or another has generally to be applied to those steel stocks, despite considerable increase in price, to make the strength, ductility, toughness and weldability all adequate for a structure member steel.

Thus, an object of this invention resides in the provision of inexpensive, weldable, tough and high strength steel for structure members that is perfectly usable in its as-hot-worked state, namely without any subsequent or independent heat treatment, having a set of mechanical properties which is characterized by strength, ductility and toughness all maintained at a high level. Another object is to determine the proper composition for the steel described above so that the steel may possess the set of mechanical properties as desired in the as-hotworked state when manufactured properly. The third is to give the proper method of hot-working for the particular steel described above, by which combination, namely proper hot-working on steel of proper composition, it is only possible to obtain the set of mechanical properties desired. Other objects of this invention will in part be mentioned in due course, and yet others will either be self-explanatory or be readily understood or appreciated by those who are skilled in the art.

More specifically, this invention relates to a weldable, tough and high strength steel for structure member that is capable of developing a tensile strength of at least 58 kg./mm. but not over 70 kg./mm. yield ratio, the ratio of yield strength to tensile strength, of at least 72%, a ductility of at least 25% in elongation on gauge length of four times the specimen diameter at room temperature, and a notch impact toughness of at least 80 ft.-1b. or 13.8 kg.-m./cm. at 0 C. by 2 mm. V notch Charpy test piece.

It also relates to a steel composition in the final product analysis which comprises by weight percent: carbon 0.08- 0.30%, silicon 0.02-0.60%, manganese 0.52.50% nitrogen less than 0.045%, the whole or at least 0.010% of which is to be bound to aluminum in the form of precipitated aluminum nitride of 0.03-0.12%, the rest to any one or combination selected from the group composed of: columbium less than 0.20%, titanium less than 0.12% and zirconium less than 0.20%: free uncombined nitrogen less than 0.004%, free metallic aluminum dissolved in matrix as solid solution (solid solution metallic aluminum) less than 0.15%, as well as at least one complementing alloying element selected from the group consisting of: nickel 0.101.0%, chromium 0.10l.0% molybdenum 0.051.0%, copper ODS-1.0%, vanadium ODDS-0.30%, boron 0.00050.01%; the balance substantially all iron with incidental or unavoidable impurities.

This invention relates further to a method of manu tacture for weldable, tough and high strength steel or structure member characterized by the set of mechanical properties defined earlier in the as-hot-worked state, the method comprising hot-working the steel in any manner in the temperature range of 1200700 C., starting at. a temperature not above 1200 C. and finishing at not below 700 C., giving the steel a reduction in thickness, calculated on the final thickness of the product, of at least 20% in the temperature range below 1000 C. in one or more passes, thereafter cooling the steel.

As mentioned earlier briefly, in order to obtain a set of mechanical properties which is as good as or even better in the as-hot-worked state than that may be realized after heat treatment, not only the hot-working procedure but also the composition of the steel stock must be carefully controlled. Namely, this invention is based on the discovery of the conditions governing 'both those ends. In the steel composition: Carbon, the foremost alloying element for steel, is necessary by at least 0.08% to ensure the 60 kg./mm. class tensile strength when co-prescribed with other ingredients, but tends to spoil the weldability of the steel when present in excess of 0.30%, the preferred range falling in from about 0.10% to 0.25%.

Silicon is added as deoxidizing agent for which at least 0.02% is needed, but tends to excessively harden the matrix by solid solution effect and to affect unfavorably the ductility when present by more than 0.60%, the preferable range being from about 0.3 to 0.5%.

Manganese, which is also an active deoxidizer, is an element that is capable of hardening the steel without impairing the weldability, for which purpose at least 0.5% is needed. However, when present in excess of 2.5%, it tends to stabilize the lower baim'te in the as-hot-worked steel to give rise to a higher strength at a considerable expense in the ductility and toughness. Thereupon, the manganese content is limited to from 0.5 to 2.5%, of which 0.5 to 2.0% is the preferred range.

Aluminum nitride component of at least 0.03% on nitrogen of about 0.01% is necessary to ensure a high ductility and toughness. However, we have observed in attempting to provide precipitated aluminum nitride more than about 0.12%, or, in terms of nitrogen bound to aluminum, 0.041%, the nonmetalli-c inclusions, particularly oxides of aluminum and silicon that are inevitably retained in the steel, increases rapidly and the cieanness, and with it the ductility and toughness, of the steel correspondingly declined. The favorable effect the precipitated aluminum nit-ride exerts on the toughness and ductility itself, on the other hand, tends to saturate as the content approaches this 0.12% level.

Some solid solution metallic aluminum content is unavoidable insofar as more aluminum must be provided over what is needed for the stoichiometric combination with available nitrogen to ensure the desired aluminu r n nitride content, but it is an unwelcome constituent for the steel because aluminum dissolved in solid solution with the matrix tends to coarsen the grain and thence to affect adversely the mechanical properties, particularly the low temperature toughness, of the steel. We have found that it is allowable only to 0.15% in the steels of this invention.

Of other metallic nitrides than that of aluminum, we found those of columbium, titanium and zirconium are effective for this invention when existing in conjunction with aluminum nitride without causing the oxide or grain coarsening trouble of the former. For this purpose, columbium less than 0.20%, titanium less than 0.12%, zirconium less than 0.20% may be provided. However, when present in excess, they tend to form oxide and carbide which would act as the crack formation site, and the toughness of the steel is adversely aifected. Free uncombined nitrogen is again unavoidable and on the whole unwelcome, the less of it being the better, for its unfavorable etfect on the low temperature toughness completely overshadows its favorable effect on the yield point. However, with an aluminum nitride content of more than 0.03%, free uncombined nitrogen of up to 0.004% may be allowed for steel of this invention. The provision of columbium, titanium and zirconium helps to reduce the free uncombined nitrogen further, and particularly with the last two, the content of the latter can be reduced. to the order of or less than 0.001%.

The above-listed elements, particularly carbon, silicon, manganese, and aluminium nitride, are the fundamental constituents of steel according to the invention. However, a satisfactory combination of high strength and good toughness cannot always be guaranteed by carbon-siliconmanganese-aluminium nitride combination alone if the size, e.g. the thickness of the product becomes large, say more than one-half inch.

Here resides the significance of other alloying element or elements that complement this shortcoming of the fundamental composition. Namely, we have found that any one or combination of the following elements is further necessary for the purpose of this invention already set forth.

Nickel is useful to increase the strength and particularly the toughness of the steel as well as to counter the hot shortness caused by the copper component, when present, and indeed the more of it, the better. However, considering the price with regard to the gain in mechanical properties, a content between 0.10 to 1.0%, preferably about 0.2 to 0.8% was found to be proper.

Chromium is advantageous to ensure a high strength in the as-hot-worked state, though it tends to adversely affect the toughness, hence a range of 0.10 to 1.0%, preferably 0.3 to 0.8%, is useful.

Molybdenum, like chromium, helps to strengthen the steel but also tends to impair the toughness, hence a range of 0.05 to 1.0%, preferably 0.1 to 0.6%, should be present.

Copper is effective to improve the corrosion resistivity as well as to raise the strength level of the steel, but tends to develop hot shortness and to affect unfavorably on the weldability and low temperature toughness, hence we use a range of 0.05 to 1.0%, preferably 0.1 to 0.8%.

Vanadium also strengthens the steel when dissolved. in the matrix as solid solution, but tends to enhance the weld-cracking trouble. Balancing those against its price, a range of 0.005 to 0.30%, preferably 0.02 to 0.1%, was found to be satisfactory.

Boron imparts additional strength without impairing the toughness or weldability in a range of 0.0005 to 0.01%, preferably 0.0005 to 0.005%.

We observed that even those steels composed carefully according to above-described principles did not come up with the expected set of mechanical properties defined earlier if hot-worked in a commonly employed manner, for example, rolled in the range of 1300 to "900 C., starting in the vicinity of the former and finishing close to the latter, the overall reduction in thickness 'being distributed more or less evenly between passes. We have traced this failure to the precipitation characteristic of the metallic nitride component, particularly aluminum nitride, and to the unsatisfactory state of dispersion thereof. After many tests and experiments, we have found that not only the hot-working temperature range but also the working schedule must be specified. Namely, in order for steels of this invention to develop the set of mechanical properties specified, the temperature range in which to hot-work must be 1200 to 700 C., that is to say, the starting temperature not above 1200 C. and finishing temperature not below 700 C., and the hotworking pass schedule such that a reduction in thickness, calculated on the final thickness of the product, of at least 20% be given in one or more passes which are to be conducted at a temperature below 1000 C. Thereafter the steel thus prepared maybe cooled in any manner, for example, it may be left in air.

Although the exact reason for this difference has not been explained, I believe that the nitride particles, which have already precipitated during the prior stages of steelmaking, comprising the ingot making and blooming, play an important role. Therefore, the soaking temperature, i.e. the starting temperature for hot-working must not be so high as to cause the dissolution of nitride precipitates; a temperature range of 1100 to 1200 C. has been found proper with regard to the preservation of once precipitated nitride particles and to the ease of hot-working as well.

The details and merits of this invention will further be apparent from the example described below in which a number of steels of this invention or those acquired on the commercial market were hot-worked by rolling as indicated.

Example to this invention method, and Steel 11 according to common practice, while Table 3 summarizes the mechanical properties of product steels.

TABLE 1.CHEMICAL COMPOSITION BY PRODUCT ANALYSIS (WT. PERCENT) Steel C Si Mn Ni Cl Mo Cu V Cb Ti Zr 13 AlN 0. 18 0. 40 1. 43 O. 052 0. 16 O. 40 1. 44 0. 032 0. 19 0. 4O 1. 50 0. 055 0. 20 0. 40 1. 51 O. 052 0. 18 0. 40 1. 28 0. 036 0. 17 0. 4O 1. 23 0. 049 0. 18 0. 40 l. 46 0. 055 0. 19 0. 41 1. 50 0. 052 0. 18 0. 19 0. 53 0. 047 0. 17 0. 26 0. 52 0. 003 0. 18 0. 56 1. 48 0. 005

TABLE 111 Al in Nin Nin No. AlN Met. Al Nitride Former Other than Al .AlN Tot. N F. N AlN Other Nitride 0.052 0.051 0.035 0.021 0.004 0.017 0.032 0.033 0.021 0.014 0.003 0.011 0.055 0. 003 ob, 0.019 0.037 0. 021 0.001 0. 013 0.002 0.052 0.010 Zr, 0.03s 0.035 0.024 0. 002 0.017 0. 005 0.035 0.023 Zr,0047;B 0001- 0.024 0.017 0.001 0.012 0.004 0. 049 0.034 Zr, 0.033 0. 021 0.003 0.016 0.002 0.055 0.019 v, 0.04; Ti, 0. 0.037 0.022 0.002 0.013 0.002 0.052 0. 012 v, 0.04; Cb, 0.04 0.035 0.024 0. 002 0.017 0.005 0.047 0. 013 Cb, 0.041; Zr, 0.093; B,0.001 0.032 0.023 0.000 0.015 0.008 0.003 0.031 0. 002 0. 003 0.007 0. 001 11 0.005 0.000 0.003 0.007 0.005 0.002

TABLE ROLLING DATA Although only the fiat-rolling process was used in describing the method of the invention, we hold no special Smking Rolling Schedule preference for it in the hot-working method. Forging, Steel Tefilpem swaging, section-rolling or any other known processes of ture, 0. Starting Finishing Finishing h t. r

Tempew Tempem Reduction work ng a e equally sat1stactory nasmuch as the ture, 0. ture, 0. Per ent workrng is carried out in accordance w1th the method of this invention. 1,150 1,100 850 40 Having described heretofore the principles and applica- 22g t10ns of this invention, I do not wish to be confined to the 1:160 1:100 840 50 factual examples shown, but only by the claims as set 1,150 1,110 840 45 fo th 1,150 1, 090 800 25 1,150 1, 000 320 34 I 1.140 1,100 49 1. A method of manufacturing weldable, tough and 1, 100 1, 070 190 44 1,140 1,100 800 41 high strength steel for structure members usable in the 1.320 1,270 950 10 n0 as-hot-worked state, said steel consisting of carbon 0.08- 0.30%, silicon 0.02-0.60%, manganese 0.5-2.5 nitro- 1 Reduction in thickness calculated on product, obtained at temperatures below 1,000 C.

TABLE 3.MECHANICAL PROPERTIES OF THICK PLATE Yield Tensile ElonvEn, Yield Steel Strength, Strength, gation, kg.-m/cm. Ratio,

lrglmm. kg./1nm. Percent Percent 1 Gauge length mm., 10 mm. diameter specimen. 2 Charpy 2 mm. V notch specimen, at 0 C.

It will be seen in those tables that the steels with chemical composition according to this invention and prepared in accordance with the method of the invention all exhibit the set of mechanical properties specified. It is further noted that Steel 10, which was prepared according to the method of the invention from commercial Cr-Mo type steel stock, compares very favorably with Steel 11, which is in fact a high quality low-manganese grade commercial steel. However, the low temperature notch toughness of Steel 10 fails to attain the 80 ft.-lb. (13.8 kg.-m/cm. level, or the yield ratio the 72% requirement, because the aluminum nitride content is not as much as required in this invention. Those results shown by Steels 1 to 9 of this invention are simply compared with those of Steels 10 and 11 for the superiority to be appreciated.

It will further be seen that a wide variety of combina- :tions of mechanical properties fit for particular service requirements can be gained by properly selecting the chemical composition and/or manufacturing procedures. For example, Steels 1, 3 and 8 may advantageously be used for structures designed on the yield stress basis for their high yield points; here the presence of columbium, partly in the form of columbium nitride existing along with aluminum nitride, is appreciated, while the application of Steels 3, 7, 8 and 9 may be quite profitable in the low temperature service structures for their high notch toughness.

gen less than 0.045%, at least 0.010% of which is to be bound to aluminum in the form of precipitated aluminum nitride of 0.03-0.12%, the rest to any one selected from the group composed of: columbium less than 0.20%, titanium less than 0.12% and zirconium less than 0.20%, and a combination thereof; free uncombined nitrogen less than 0.004%, free metallic aluminum dissolved in the matrix as solid solution less than 0.15 as well as at least one complementing alloying element selected from the group consisting of: nickel 0.10-1.0%, chromium 0.10-1.0%, molybdenum ODS-1.0%, copper ODS-1.0%, vanadium 0.005-0.30%, boron 0.0005-0.01%; the balance substantially all iron with incidental and unavoidable impurities, the method consisting of heating the steel to a soaking temperature in the range of 1100-1200 C., hot-Working the steel in the temperature range of 1200 700 C., starting the hot working at a temperature not above 1200" C. and finishing at a temperature not below 700 C., giving the steel a reduction in thickness, calculated on the final thickness of the product, of at least 20% in at least one pass in the temperature range below 1000" C., and thereafter cooling the steel.

2. The method according to claim 1, wherein the steel consists of carbon 0.10-0.25%, silicon 0.3-0.5%, manganese 0.5-2.0%, precipitated aluminum nitride 0.03- 0.l2%, metallic aluminum dissolved in the matrix as solid solution less than 0.15%, at least one from: the group consisting of columbium less than 0.20%, titanium less than 0.12%, zirconium less than 0.20%, free uncombined nitrogen less than 0.004%, as well as at least one from: the group consisting of nickel 0.2-0.8%, chromium 03-08%, molybdenum 0.1-0.6%, copper 0.1-0.8%, vanadium 0.02O.1%, boron '0.0005-0.005%; the balance substantially all iron with incidental and unavoidable impurities.

3. A weldable, tough and high strength steel for structure members usable in the as-hot-Worked state having a tensile strength of at least 58 kg./mm. but not over 7 kg./rnm. a yield ratio of at least 72%, a ductility at room temperature of at least 25% as measured on guage length of four times the specimen diameter, and a notch impact toughness of at least ft.-lb. or 13.8 kg.-m./cm. at 0 C. as measured by 2 mm. V notch Cha-rpy test piece, said steel consisting of the ingredients and having been made by the steps defined in claim 1.

7 8 4. A weldable, tough and high strength steel for struc- 3,155,496 l1/ 1964 Nakamura 75-124 ture members usable in the as-hot-worked state having 3,155,549 11/1964 Nakamura 75l24 X a tensile strength of at least 58 kg./mrn. but not over 70 3,173,782 3/1965 Melloy t v 1 7 5 123 kg./mm. a yield ratio of at least 72%, a ductility at 3 259 3 7 19 Nakamura 75.424 room temperature of at least 25% as measured on guage 5 length of four times the specimen diameter, and a notch FOREIGN PATENTS impact tou hness of at least 80 ft.-1b. or 13.8 kg.-m./cm. at 0 C. a: measured by 2 mm. V notch Charpy test 786993 11/1957 Great i i piece, said steel consisting of the ingredients and having 8081556 2/1959 Great Bmam' been made by the steps defined in claim 2. 10 830,669 3 1960 Great i i 904,886 9/1962 Great Bntam. References Cited UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

3,010,822 11/1961 Altenburger et a1 75-423 3,155,495 1l/1964 Nakamura 75124 15 SAITO, Assistant Exammer,

Claims (1)

1. A METHOD OF MANUFACTURING WELDABLE, TOUGH AND HIGH STRENGTH STEEL FOR STRUCTURE MEMBERS USABLE IN THE AS-HOT-WORKED STATE, SAID STEEL CONSISTING OF CARBON 0.080.30% SILICON 0.02-0.60%M MANGANESE 0.5-2.5%, NITROGEN LESS THAN 0.345%, AT LEAST 0.010% OF WHICH IS TO BE BOUND TO ALUMINUM IN THE FORM OF PRECIPITATED ALUMINUM NITRIDE OF 0.03-0.12%, THE REST TO ANY ONE SELECTED FROM THE GROUP COMPOSED OF: COLUMBIUM LESS THAN 0.20%, TITANIUM LESS THAN 0.12% AND ZIRCONIUM LESS THAN 0.20%, AND A COMBINATION THEREOF; FREE UNCOMBINED NITROGEN LESS THAN 0.004%, FREE METALLIC ALUMINUM DISSOVED IN THE MATRIX AS SOLID SOLUTION LESS THAN 0.15%, AS WELL AS AT LEAST ONE COMPLEMENTING ALLOYING ELEMENT SELECTED FROM THE GROUP CONSISTING OF: NICKEL 0.10-1.0%, CHROMIUM 0.10-1.0%, MOLYBDENUM 0.05-1.0%, COPPER 0.05-1.0%, VANADIUM 0.005-0.30%, BORON 0.0005-0.01%; THE BALANCE SUBSTANTIALLY ALL IRON WITH INCIDENTAL AND UNAVOIDABLE IMPURITES, THE METHOD CONSISTING OF HEATING THE STEEL TO A SOAKING TEMPERATURE IN THE RANGE OF 1100*- 1200*C., HOT-WORKSING THE STEEL IN THE TEMPERATURE RANGE OF 1200*700*C., STARTING THE HOTWORKING AT A TEMPERATURE NOT ABOVE 1200*C. AND FINISHING AT A TEMPERATURE NOT BELOW 700*C., GIVING THE STEEL A REDUCTION IN THICKNESS, CALCULATED ON THE FINAL THICKNESS OF THE PRODUCT, OF AT LEAST 20% IN AT LEAST ONE PASS IN THE TEMPERATURE RANGE BELOW 1000*C., AND THEREAFTER COOLING THE STEEL.